that naked nigh > [so that almost naked]
3 With ghastly looks and dreadful drearihead:
ghastly > {Frightful, evoking terror of the supernatural} looks > appearance (as in "good looks"); glances drearihead > dreariness, melancholy
4 For from her back her garments she did tear, 5 And from her head oft rent her snarled hair;
rent > tore
6 In her right hand a firebrand she did toss
toss > shake, brandish
7 About her head, still roaming here and there:
still > continually
8 As a dismayed deer, in chase embossed,